Various image reading apparatuses such as scanners include a device with particularly high resolutions (hereinafter referred to as a high-resolution scanner). Such a high-resolution scanner can typically switch among several resolutions, for example, 700 dpi, 2400 dpi, and 4000 dpi. Since each of these three resolutions requires a lens for reading with its resolution, three lenses are required in a case that the resolutions are switched with such three levels, for example. The price of a lens usually increases with resolution. In addition, switching of resolutions results in reduction in its readable region. The resultant readable region is limited to a central portion in the width direction (main scanning direction) of its document table (platen glass).
As described above, the high-resolution scanner requires the same number of lenses as that of resolutions to be switched. The lenses are very expensive and then the high-resolution scanner is also expensive. In particular, this tendency may become more pronounced in a scanner that provides higher resolutions in multiple numbers.
As also described above, switching of resolutions will limit the readable region to a central portion in the main scanning direction. Thus, all image data cannot be acquired throughout the whole range covered by the whole document table (that is, the whole image reading region).
It is an object of the present invention to provide a high-resolution image reading apparatus which can read an image with fewer lenses and more resolutions.